Concrete is a composite material consisting of a mineral-based hydraulic binder which acts to adhere mineral particulates together in a solid mass; those particulates may consist of coarse aggregate (rock or gravel), fine aggregate (natural sand or crushed fines), and/or unhydrated or unreacted cement. Concrete typically is made from portland cement (“PC”), water and aggregate. Curing concrete requires two elements: suitable temperature and water. To achieve maximum strength, all cement particles must be hydrated. The initial process of hydration is exothermic; it generates a considerable amount of energy called “heat of hydration.” Fluid (plastic) concrete is poured in various forms or molds and left to set until it has hardened sufficiently to remove the concrete forms. These prior art forms are not insulated and therefore concrete is exposed to the environment. Consequently, the energy generated from the heat of hydration is generally lost to the environment in the first 12-20 hrs. Generally, the concrete forms are removed exposing the concrete to the environment. In the next few days, most of the initial concrete moisture is also lost from the concrete. Therefore, the two elements required to fully hydrate the cement are lost during the initial stage of concrete curing. Thus, the cement may never fully hydrate, and, therefore, may never achieve maximum strength. Portland cement concrete achieves 90% of maximum strength under ideal curing conditions in about 28 days.
Portland cement manufacture causes environmental impacts at all stages of the process. During manufacture, a metric ton of CO2 is released for every metric ton of portland cement made. Worldwide CO2 emissions from portland cement manufacture amount to about 5-7% of total CO2 emissions. The average energy input required to make one ton of portland cement is about 4.7 million Btu—the equivalent of about 418 pounds of coal. The production of portland cement is energy intensive, accounting for 2% of primary energy consumption globally. In 2010 the world production of hydraulic cement was 3,300 million tons.
Concrete can also be made with slag cement (“SC”) and fly ash (“FA”) but are not frequently used. Slag cement and fly ash generate relatively low amounts of heat of hydration, which result in extremely slow setting time and strength gain. Slag cement and fly ash can be mixed with portland cement but industry practice in building construction limits use of slag cement and fly ash to no more than 30% replacement of portland cement and only during warm weather conditions. Concrete made with slag cement and fly ash may take up to 90 days to achieve 80-90% of maximum strength. Mass concrete structures use more slag cement and fly ash, replacing up to 80% of portland cement, as a means to reduce the heat of hydration to reduce cracking. Slag cement and fly ash use less water to hydrate, may have finer particles than portland cement and produce concretes that achieve higher compressive and flexural strength. Such concrete is also less permeable, and, therefore, structures built with slag cement and fly ash have far longer service lives.
Slag cement is obtained by quenching molten iron slag (a by-product of iron and steel-making) from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder. Slag cement manufacture uses only 15% of the energy needed to make portland cement. Since slag cement is made from a waste materials; no virgin materials are required and the amount of landfill space otherwise used for disposal is reduced. For each metric ton of pig iron produced, approximately ⅓ metric ton of slag is produced. In 2009, worldwide pig iron production was 1.211 billion tons. There was an estimated 400 million tons of slag produced that could potentially be made into slag cement. However, only a relatively small percentage of slag is used to make slag cement in the USA.
Fly ash is a by-product of the combustion of pulverized coal in electric power generation plants. When pulverized coal is ignited in a combustion chamber, the carbon and volatile materials are burned off. However, some of the mineral impurities of clay, shale, feldspars, etc. are fused in suspension and carried out of the combustion chamber in the exhaust gases. As the exhaust gases cool, the fused materials solidify into spherical glassy particles called fly ash. The quantity of fly ash produced is growing along with the steady global increase in coal use. According to Obada Kayali, a civil engineer at the University of New South Wales Australian Defense Force Academy, only 9% of the 600 million tons of fly ash produced worldwide in 2000 was recycled and even smaller amount used in concrete; most of the rest is disposed of in landfills. Since fly ash is a waste product, no additional energy is required to make it.
Historically, concrete has been made using natural cements and other pozzolanic materials, such as volcanic ash, certain type of reactive clays, rice husk ash, metakolin, silica fume and others. Pozzolanic materials have a very low rate of hydration and generate less heat of hydration. Therefore concrete made with pozzolanic materials are seldom used due to their slower curing process.
More recently pozzolanic materials, such a fly ash and volcanic ash have been modified through a process of fracturing which produces what is called “energetically modified cement.” Such pozzolanic materials are typically of a generally spherical shape but can be fractured so that the round sphere particle is broken up into multiple particles with more surface area. The greater surface area creates a higher reactive particle, therefore increasing the hydration properties of the pozzolanic material.
Concrete walls, and other concrete structures and objects, traditionally are made by building a form or a mold. The forms and molds are usually made from wood, plywood, metal and other structural members. Unhardened (plastic) concrete is poured into the space defined by opposed spaced form members. Once the concrete hardens sufficiently, although not completely, the forms are removed leaving a concrete wall or other concrete structure, structural member or concrete object exposed to ambient temperatures. Concrete forms are typically made of various types of plywood or metal supported by a frame. These forms are not insulated which means that concrete is exposed to the elements during the initial portion of the curing process. This often makes the curing of the concrete a slow process and the ultimate strength difficult to control or predict. To compensate for these losses and increase the rates of setting and strength development, larger amounts of portland cement are used than otherwise would be necessary.
The curing of plastic concrete requires two elements, water and heat, to fully hydrate the cementitious material. The curing of plastic concrete is an exothermic process. This heat is produced by the hydration of the portland cement, or other pozzolanic or cementitious materials, that make up the concrete. Initially, the hydration process produces a relatively large amount of heat. Concrete placed in conventional forms (i.e., uninsulated forms) loses this heat of hydration to the environment in a very short time, generally in the first 8-24 hours, depending on the ambient temperature. Also, due to the heat loss to the environment the concrete placed in conventional forms does not reach its maximum potential temperature. As the hydration process proceeds, relatively less heat of hydration is generated due to slowing reaction rates. At the same time, moisture in the concrete is lost to the environment. If one monitors the temperature of concrete during the curing process, it initially produces a relatively large increase in temperature which then decreases relatively rapidly over time. This chemical reaction is temperature dependent. That is, the hydration process, and consequently the strength gain, proceeds faster at higher temperature and slower at lower temperature. In conventional forms, both heat and moisture are lost in a relatively short time, which makes it difficult, or impossible, for the cementitious material to fully hydrate, and, therefore, the concrete may not achieve its maximum potential strength.
Concrete in conventional concrete forms or molds is typically exposed to the elements. Conventional forms or molds provide little or no insulation to the concrete contained therein. Therefore, heat produced within the concrete form or mold due to the hydration process usually is lost through a conventional concrete form or mold relatively quickly to the environment. Thus, the temperature of the plastic concrete may initially rise 20 to 40° C., or more, above ambient temperature due to the initial hydration process and then fall relatively quickly to ambient temperature, such as within 8 to 36 hours depending on the climate and season and size of the concrete element. This initial relatively large temperature drop may result in significant concrete shrinkage and/or thermal effects which can lead to concrete cracking. The remainder of the curing process is then conducted at approximately ambient temperatures, because the relatively small amount of additional heat produced by the remaining hydration process is relatively quickly lost through the uninsulated concrete form or mold. The concrete is therefore subjected to the hourly or daily fluctuations of ambient temperature from hour-to-hour, from day-to-night and from day-to-day. Failure to cure the concrete under ideal temperature and moisture conditions affects the ultimate strength and durability of the concrete. In addition the temperature gain and loss in the first few days of concrete curing creates thermal stresses within the concrete. At the time that the concrete reaches its maximum temperature, usually 8-16 hrs, the concrete is in a relatively weak state and cannot withstand the thermal stresses very well. The cooling of the concrete from the initial temperature peak creates temperature shrinkage cracking within the cement paste. The further heat loss and gain due to the ambient temperature fluctuations from day and night places additional thermal stresses upon the concrete and further contributes to temperature shrinkage cracking. While initially temperature shrinkage cracking is on a nano scale, with time, the nano cracks develops into fractures that weaken the concrete and shorten its lifespan.
In colder weather, concrete work may even come to a halt since concrete will freeze, or not gain much strength at all, at relatively low temperatures. By definition (ACI 306), cold weather conditions exist when “ . . . for more than 3 consecutive days, the average daily temperature is less than 40 degrees Fahrenheit and the air temperature is not greater than 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Therefore, in order for hydration to take place, the temperature of concrete must be above 40° F.; below 40° F., the hydration process slows and at some point may stop altogether. Under conventional forming and curing methods, the concrete takes a relatively long time to fully hydrate the cementitious materials. Since both the initial heat and moisture are quickly lost in conventional forms, it is typically recommended that concrete by moisture cured for 28 days to fully hydrate the concrete. However, moisture curing for 28 days is seldom possible to administer in commercial practice. Therefore, concrete poured in various applications in conventional forms seldom develops it maximum potential strength and durability.
Insulated concrete form systems are known in the prior art and typically are made from a plurality of modular form members. U.S. Pat. Nos. 5,497,592; 5,809,725; 6,668,503; 6,898,912 and 7,124,547 (the disclosures of which are all incorporated herein by reference in their entirety) are exemplary of prior art modular insulated concrete form systems. Full-height insulated concrete forms are also known in the prior art. U.S. Patent Application Publication No. 2013/0074432 and U.S. Pat. No. 8,555,583 (the disclosures of which are both incorporated herein by reference in their entirety) disclose full-height insulated concrete forms. However, these insulated concrete forms are stay in place concrete forms whereby the insulating panels are attached to the concrete and are not easily removed. In addition if these insulated panels are removed from the concrete, they are usually damaged and not able to be reused.
Although insulated concrete forms work well and provide many benefits, concrete contractors and architects are somewhat reluctant to use them or specify them. Especially, stay in place insulated concrete forms cannot be used for applications that require removal of the formwork. Under conventional forming and curing methods, the concrete takes a relatively long time to fully hydrate the cementitious materials. Since both the initial heat and moisture is often relatively quickly lost, it is typically recommended that concrete be moist cured for 28 days to fully hydrate the cement. However, moisture curing for 28 days is seldom possible to achieve in commercial practice. Therefore, for concrete poured for various applications it can be very difficult, or impossible, to achieve its maximum potential strength and durability. Current insulated concrete forms are made of polymeric foam and remain in place after concrete is placed. However, there are many types of applications that do not need the insulation provided by insulated concrete forms to remain in place as part of the structure.
It is believed that prior art concrete forms have not been proposed or used as a method to cure concrete or to improve the performance and properties of concrete. The present invention has discovered that when retaining in an insulated concrete form the initial heat generated by the hydration of cementitious material, the concrete achieves a greater internal temperature and such temperature is sustained for much longer periods of time before it is lost to the environment. During this time, there is sufficient moisture in the concrete to hydrate the cementitious material. When the insulated concrete forms are removed, usually a few days after the pour, the concrete and cement paste would have already achieved a relatively high level a cement hydration with a relatively high corresponding compressive strength. A more fully hydrated cement paste and higher strength concrete is better able to withstand the stresses associated with temperature loss. Thus, the inevitable temperature shrinkage cracking associated with concrete forming is greatly reduced or eliminated.
Many concrete contractors prefer to use the prior art plywood-type form board and frame concrete form because it is the form with which they and the construction workforce are familiar. Therefore, it would be desirable to produce a concrete form that combines the benefits of an insulated concrete form with a removable conventional concrete form frame type that can retain the initial heat of hydration to accelerate the hydration and curing process and more fully cure concrete immediately after concrete is placed in the forms. It also would be desirable to reduce or eliminate temperature shrinkage cracking associated with conventional concrete forming. Any type of concrete placed in such forms will have far greater and improved properties and be more durable and longer lasting. It is also desirable to make concrete from as much post industrial waste as possible thereby reducing the burden on landfill. It would also be desirable to reduce the amount of portland cement used in concrete as much as possible to thereby reduce the amount of CO2 emissions associated with manufacture of portland cement.